1. Field of the Invention
The present invention relates to methods of manufacturing an image device. More particularly, the present invention relates to methods of manufacturing a complementary metal-oxide semiconductor (CMOS) image sensor.
2. Description of the Related Art
An image sensor may convert optical information of two or more dimensions into an electrical signal. The image sensor may be classified as either an image pick-up tube or a solid-state imaging device. Image pick-up tubes are widely employed in television cameras. In addition, recent advancements in image processing technology of the image pick-up tube have been developed. Examples of the image processing technology include image measurement, image control and image recognition. The solid-state imaging device may be classified as a complementary metal-oxide semiconductor (CMOS) image sensor or a charge-coupled device (CCD).
The CMOS image sensor may convert an optical image into an electrical signal by using a photodiode and a metal-oxide semiconductor (MOS) transistor.
The CMOS image sensor was invented in the 1960s. However, until recently, image quality of the CMOS image sensor was inferior to that of the CCD because of noise such as fixed pattern noise (FPN). In addition, a circuit included in the CMOS image sensor was more complex than that of the CCD. Furthermore, a packing density of the CMOS image sensor was lower than that of the CCD. However, a cost required for manufacturing the CMOS image sensor was substantially the same as that of the CCD. In addition, a size of the CMOS image sensor was relatively larger. Thus, the CMOS image sensor was hardly developed until the 1990s.
However, some disadvantages of the CMOS image sensor started to be overcome in the 1990s by development of CMOS processing technology and improvement of signal processing algorithms in the late 1990s. In addition, some characteristics of the CCD were applied to the CMOS image sensor so that image characteristics of the CMOS image sensor were improved.
The CMOS image sensor is advantageous in that the CMOS image sensor operates with relatively low power. In addition, the CMOS image sensor is capable of allowing random access to image data in pixel regions. Furthermore, the CMOS image sensor is advantageous in that costs for manufacturing the CMOS image sensor may be reduced by employing general CMOS processes.
Currently, because an image sensor such as that of a digital still camera, a camera of a cellular phone or a camera of a door phone is being widely used, the CMOS image sensor is much in demand. Thus, a highly effective CMOS image sensor that can be used in a wide variety of applications is being largely researched.
The CMOS image sensor has a relatively small size and a fine design rule. Thus, in case that aluminum wires are employed in the CMOS image device, processes for manufacturing the CMOS image device may be relatively difficult, compared to employing copper wires. Since copper is a better conductor than aluminum, thinner wires can be formed when copper is used. Accordingly, it is desired that the CMOS image device employ copper wires rather than the aluminum wires.
However, a copper layer that is patterned to form a copper wire may not be easily patterned by a reactive ion etch (RIE) process. Thus, the copper wire may be efficiently formed by a damascene process rather than the RIE process. In case that the copper wire is formed by the damascene process, an opaque capping layer may be further included in the CMOS image device. Particularly, the opaque capping layer may be formed on a transparent insulation layer included in the CMOS image device. The opaque capping layer may prevent copper included in the copper wire from being easily diffused. In addition, the opaque capping layer may serve as an etch stop layer. The opaque capping layer may include an opaque material such as silicon nitride or silicon carbide.
Because the opaque capping layer is opaque, light may hardly pass through the opaque capping layer. Thus, if a portion of the capping layer, positioned over the photodiode, is not removed, the light may not be incident onto the photodiode. If the light is not incident onto the photodiode, the image sensor may not operate.
Thus, the portions of the transparent insulation layer and the opaque capping layer, positioned over the photodiode, are removed by an etching process.
However, if the etching process is excessively performed, the photodiode may be unfortunately exposed. Thus, an exposed portion of the photodiode may be damaged.
On the other hand, if the etching process is insufficiently performed, a residual portion of the opaque capping layer may remain over the photodiode. This residual portion of the opaque capping layer may unfortunately refract the light. In addition, the residual portion of the opaque capping layer may unfortunately block the light. Consequently, the light may not be incident onto the photodiode.
Thus, the etching process has to be controlled so that the photodiode may not be exposed. In addition, the etching process has to be controlled so that the portions of the transparent insulation layer and the opaque capping layer positioned over the photodiode may be clearly removed.
However, if the CMOS image device has a multi-layered structure, there is a difference in thickness between the transparent insulation layers in the multi-layered structure. In addition, the opaque capping layer may be formed between the transparent insulation layers. Thus, it is difficult to completely remove opaque portions over the lowest transparent insulation layer in the etching process without damage to the lowest transparent insulation layer. In addition, the CMOS image device is made to be thin to improve the transmittance of the light. Thus, in case that the etching process is excessively performed, the photodiode under the lowest transparent insulation layer may be damaged as well as the lowest transparent insulation layer. On the other hand, in case that the etching process is not fully performed, the opaque portions over the lowest transparent insulation layer may partially remain. Thus, a margin of the etching process for selectively removing the opaque portions over the lowest transparent insulation layer may unfortunately decrease.